Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 28 Feb 2022 14:53:07 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        alsa-devel@alsa-project.org, KVM list <kvm@vger.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        linux-iio@vger.kernel.org, nouveau@lists.freedesktop.org,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>, linux1394-devel@lists.sourceforge.net,
        drbd-dev@lists.linbit.com, linux-arch <linux-arch@vger.kernel.org>,
        CIFS <linux-cifs@vger.kernel.org>, linux-aspeed@lists.ozlabs.org,
        linux-scsi <linux-scsi@vger.kernel.org>,
        linux-rdma <linux-rdma@vger.kernel.org>,
        linux-staging@lists.linux.dev,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        intel-wired-lan@lists.osuosl.org,
        kgdb-bugreport@lists.sourceforge.net,
        bcm-kernel-feedback-list@broadcom.com,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergman <arnd@arndb.de>,
        Linux PM <linux-pm@vger.kernel.org>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
        Nathan Chancellor <nathan@kernel.org>,
        dma <dmaengine@vger.kernel.org>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Jakob Koschel <jakobkoschel@gmail.com>,
        v9fs-developer@lists.sourceforge.net,
        linux-tegra <linux-tegra@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-sgx@vger.kernel.org,
        linux-block <linux-block@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>, linux-usb@vger.kernel.org,
        samba-technical@lists.samba.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux F2FS Dev Mailing List 
        <linux-f2fs-devel@lists.sourceforge.net>,
        tipc-discussion@lists.sourceforge.net,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-mediatek@lists.infradead.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Mike Rapoport <rppt@kernel.org>
Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr
Message-ID: <20220228205307.GD614@gate.crashing.org>
References: <20220228110822.491923-1-jakobkoschel@gmail.com> <20220228110822.491923-3-jakobkoschel@gmail.com> <2e4e95d6-f6c9-a188-e1cd-b1eae465562a@amd.com> <CAHk-=wgQps58DPEOe4y5cTh5oE9EdNTWRLXzgMiETc+mFX7jzw@mail.gmail.com> <CAHk-=wj8fkosQ7=bps5K+DDazBXk=ypfn49A0sEq+7-nZnyfXA@mail.gmail.com> <CAHk-=wiTCvLQkHcJ3y0hpqH7FEk9D28LDvZZogC6OVLk7naBww@mail.gmail.com> <CAHk-=wj27SZQ3kPTesBzkiGhe-mA3gOQqr_adt_bMFzmg1VNaA@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wj27SZQ3kPTesBzkiGhe-mA3gOQqr_adt_bMFzmg1VNaA@mail.gmail.com>
User-Agent: Mutt/1.4.2.3i
Precedence: bulk

On Mon, Feb 28, 2022 at 12:14:44PM -0800, Linus Torvalds wrote:
> On Mon, Feb 28, 2022 at 12:10 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > We can do
> >
> >         typeof(pos) pos
> >
> > in the 'for ()' loop, and never use __iter at all.
> >
> > That means that inside the for-loop, we use a _different_ 'pos' than outside.
> 
> The thing that makes me throw up in my mouth a bit is that in that
> 
>         typeof(pos) pos
> 
> the first 'pos' (that we use for just the typeof) is that outer-level
> 'pos', IOW it's a *different* 'pos' than the second 'pos' in that same
> declaration that declares the inner level shadowing new 'pos'
> variable.

The new "pos" has not yet been declared, so this has to refer to the
outer "pos", it cannot be the inner one.  Because it hasn't been
declared yet :-)

Compare this to
  typeof (pos) pos = pos;
where that last "pos" *does* refer to the newly declared one: that
declaration has already been done!  (So this code is UB btw, 6.3.2.1/2).

> If I was a compiler person, I would say "Linus, that thing is too ugly
> to live", and I would hate it. I'm just hoping that even compiler
> people say "that's *so* ugly it's almost beautiful".

It is perfectly well-defined.  Well, it would be good if we (GCC) would
document it does work, and if someone tested it on LLVM as well.  But it
is really hard to implement it to *not* work :-)

> Because it does seem to work. It's not pretty, but hey, it's not like
> our headers are really ever be winning any beauty contests...

It is very pretty!  Needs a comment though :-)


Segher